In boundary changes proposed by a review in 1995, the seat was originally to be abolished and replaced by a new Mid Down constituency. This provoked a storm of protest and following a local enquiry minor changes were made with the seat losing one small section to Lagan Valley and another to Strangford. It still consists of parts of Down, Banbridge and Newry and Mourne districts.

In 2005, the Boundary Commission published provisional recommendations for modifying the boundaries of constituencies in Northern Ireland. For South Down, it originally proposed to add part of Newry from Newry and Armagh and the Loughbrickland part of Banbridge district from Upper Bann, while losing some more of Down to Strangford. These changes were challenged in a round of public consultations, with the result that revised recommendations were made. Under the new proposals, the Newry area remained in Newry and Armagh and Loughbrickland in Upper Bann. This meant that only 4 wards around the town of Ballynahinch were transferred to Strangford. These changes became the final recommendations and were given legal effect in 2008.

The constituency was a predominantly Nationalist area in 1918. The Unionists had significant but minority support. The Sinn Féin candidate polled poorly, probably due to the limited electoral pact to avoid seriously splitting the anti-unionist vote in seats the unionist candidate might have otherwise won.

Sinn Féin contested the general election of 1918 on the platform that instead of taking up any seats they won in the United Kingdom Parliament, they would establish a revolutionary assembly in Dublin. In republican theory every MP elected in Ireland was a potential Deputy to this assembly. In practice only the Sinn Féin members accepted the offer.

The revolutionary First Dáil assembled on 21 January 1919 and last met on 10 May 1921. The First Dáil, according to a resolution passed on 10 May 1921, was formally dissolved on the assembling of the Second Dáil. This took place on 16 August 1921.

When initially created this seat had a clear unionist majority, albeit with a strong nationalist minority. However boundary changes, which have wrapped it closer around nationalist heartlands near Downpatrick and the Mournes have transformed South Down into a safe nationalist seat.

Powell advocated a policy of integration for Northern Ireland whereby all forms of devolution would be wound up and the province governed as an integral part of the United Kingdom. As part of this he campaigned for the province to have the same ratio of MPs to population as in the rest of the United Kingdom, rather than fewer, which had previously been justified due to the existence of the devolved Stormont Parliament. Powell was successful in this but a side effect was that in his own constituency a significant block of unionist voters were removed, resulting in a nationalist majority. Powell managed to survive due to a split nationalist vote, but in 1987 he narrowly lost to Eddie McGrady of the Social Democratic and Labour Party, who held the seat until retiring in 2010.

Since then the unionist vote has declined further due to boundary changes, which excluded mainly unionist Dromore and Saintfield, and a trend for many unionists to tactically vote for the SDLP at Westminster elections to avoid the seat falling to Sinn Féin.